Because conventional amalgam restorations rely purely on mechanical retention to the tooth, they are not suitable in instances when a tooth is severely decayed and has little structure left to which the filling may be attached. Amalgam is created by mixing (amalgamation or trituration) mercury with what is often referred to as amalgam alloy. Amalgam alloy is a powder composed of various alloyed metals and is primarily composed of a silver-tin alloy. When mercury is mixed with the silver-tin alloy, a silvery paste results which is applied for condensation into the prepared lesion or cavity. However, a conventional amalgam restoration in a tooth with even only a moderate amount of decay may fail over time due to recurrent decay, lack of retention, or continued stress breakdown of the remaining tooth structure. Therefore, the patient is often faced with the choice of removing the tooth or the application of a crown.
In some instances, retentive pins are used to affix restorations to a tooth. However, retentive pins are an added expense, take time to clinically place, can weaken the restoration, create stresses within the tooth, and may also result in pulpal exposure. Thus, it is desirable to provide a dental restoration system which does not utilize pins or other similar mechanical mechanism so as to limit the stress placed on the tooth.
Many types of materials have been used to affix a restoration to a tooth, including glass ionomer cement and dentin bonding systems. Glass ionomer cement bonds to tooth structure dentin and has been used for restorative materials, cavity liners, bases, and crown cements. Glass ionomer cements are prepared for use, for example, by mixing a powder comprised of calcium aluminum silicate glass and a liquid comprising an aqueous solution of polyacrylic acid. As a crown cement, the glass ionomer cement is mixed, placed into the crown and, before the cement hardens, the crown is placed over the prepared tooth. As the glass ionomer cement hardens, the crown is retained on the tooth, and, after complete hardening of the cement, an excellent bond between the tooth and crown is formed. In other situations, glass ionomer cement may be applied to a tooth and allowed to harden to form a liner or base on which the amalgam may then be applied to restore the tooth using conventional techniques.
Studies have been conducted to determine the capability of glass ionomer cements to adhere to various materials, including the tests disclosed in Hotz, et al., The Bonding of Glass Ionomer Cements to Metal and Tooth Substrates, British Dental Journal, 1977; 142: 41-47. This study demonstrated that glass ionomer cement bonds well with dentin and enamel, and also adheres to some cast solid metals. Adherence to a cast solid metal is most successful when the surface of the metal is first etched with an acid, such as citric acid, before applying the glass ionomer cement. Improved adherence to etched metal indicates that adherence is primarily attributable to a mechanical, as opposed to a chemical, bond.
Results of studies such as Hotz et al., have provided a basis for the use of glass ionomer cements in various dental procedures. For example, in U.S. Pat. No. 4,654,007, a layer of glass ionomer cement is applied to a tooth before attaching a porcelain restoration. After proper hardening of the cement, the cement is etched with an acid to create microscopic surface irregularities which facilitate mechanical retention of the porcelain restoration to the tooth. The method disclosed in U.S. Pat. No. 4,738,722 is similar in that the glass ionomer cement disclosed is hardened and etched with an acid before the restoration material is placed into the cavity preparation. This method protects the pulp of the tooth by providing a layer of glass ionomer cement above the pulp.
The current restoration methods using glass ionomer cements have proven to be successful in restoring significant lesions. For example, Matis, et al., How Finishing Affects Glass Ionomers, 1991; 122: 43-46, describes a five year study to determine the effectiveness of restorations involving the use of glass ionomer cements and concluded that the glass ionomer cements are outstanding in their retentive capability. Also, researchers continue to improve the retention capability of the cements. For example, the polymerizable cement mixtures disclosed in U.S. Pat. No. 4,872,936 demonstrate increased mechanical strength, lower solubility, and exhibit no outstanding separation phenomena.
However, one shortcoming of using glass ionomer cements in present restorative techniques is the glass ionomer cement must be allowed to harden before it is etched with acid. This results in an undesirable time delay and increases the risk that damage to the tooth's nerve might occur while etching the hardened glass ionomer. Therefore, it is desirable to develop a dental restoration system which is efficient and requires relatively little time to perform.
In instances when hardened glass ionomer has been used as a base for amalgam, studies show that the glass ionomer shrinks, leaving a slight 60-80 um gap between the hardened glass ionomer and the hardened amalgam. Scherer, Reinforced Glass Ionomer Cement vs. Zinc Phosphate Cement, 18th Annual Session of the American Association for Dental Research, San Francisco, Calif. Thus, the hardened glass ionomer does not adequately bond the amalgam to the tooth.
Dentin bonding systems which utilize cements other than glass ionomer cements, such as those discussed in Johnson, et al., Dentin Bonding Systems: A Review of Current Products and Techniques, The Journal of the American Dental Association, 1991; 122: 34-41, have recently become available. The cements used in these dentin bonding systems are applied to the tooth prior to filling the tooth or to the application of a restoration. However, before being applied to the tooth, the tooth is etched with an acid to create irregularities to which the dentin bonding systems are micromechanically bonded. These systems have not been well-received due to the risk of pain and damage if the acid contacts sensitive dental nerves. In fact, Johnson et al. identifies several cautions in the use of dentin bond systems and suggests the use of a protective liner for deep lesions. Therefore, it is desirable to develop a dental restoration system which does not require etching of the tooth or of the cement.
It is also known to prepare a tooth cavity with enhanced glass ionomer cement material in which metal alloy particles are added to the glass ionomer cement. Such materials are used as a base or liner under restorations. The additive metal alloy particles may be of the same composition as is used in the powder component of a dental amalgam, that is to say, an amalgam alloy is used. However, these materials have a tendency over time to discolor the dentin and can provide an unsightly aesthetic appearance.
The invention disclosed in U.S. patent application Ser. No. 07/748,679, filed Aug. 22, 1991, now abandoned, the disclosure of which is incorporated herein by reference, comprises a dental restoration system that works well with conventional materials, specifically glass ionomer cement and amalgam, to allow the tooth to be filled instead of being extracted or requiring the application of a prosthesis such as a crown or bridge. Such a system is inexpensive to use and results in an improved bond strength over prior methods.
It is desirable to develop an amalgam-glass ionomer bonding system with an increased bond strength to ensure that such a restoration will remain intact for an extended period of time.